1. Field of the Invention
The present invention relates to a system and method for optimizing fluid flow in a pipe and in particular, fluid flow in a gas-lift well.
2. Description of Related Art
Gas-lift wells have been in use since the 1800""s and have proven particularly useful in increasing efficient rates of oil production where the reservoir natural lift is insufficient (see Brown, Connolizo and Robertson, West Texas Oil Lifting Short Course and H. W. Winkler, Misunderstood or Overlooked Gas-lift Design and Equipment Considerations, SPE, p. 351 (1994)). Typically, in a gas-lift oil well, natural gas produced in the oil field is compressed and injected in an annular space between the casing and tubing and is directed from the casing into the tubing to provide a xe2x80x9cliftxe2x80x9d to the tubing fluid column for production of oil out of the tubing. Although the tubing can be used for the injection of the lift-gas and the annular space used to produce the oil, this is rare in practice. Initially, the gas-lift wells simply I*njected the gas at the bottom of the tubing, but with deep wells this requires excessively high kick-off pressures. Later, methods were devised to inject the gas into the tubing at various depths in the wells to avoid some of the problems associated with high kick-off pressures (see U.S. Pat. No. 5,267,469).
The most common type of gas-lift well uses mechanical, bellows-type gas-lift valves attached to the tubing to regulate the flow of gas from the annular space into the tubing string (see U.S. Pat. Nos. 5,782,261 and 5,425,425). In a typical bellows-type gas-lift valve, the bellows is preset or pre-charged to a certain pressure such that the valve permits communication of gas out of the annular space and into the tubing at the pre-charged pressure. The pressure charge of each valve is selected by a well engineer depending upon the position of the valve in the well, the pressure head, the physical conditions of the well downhole, and a variety of other factors, some some of which are assumed or unknown, or will change over the production life of the well.
The typical bellows-type gas-lift valve has a pre-charge cylinder for regulating the gas flow between the annular space and the interior of the tubing string. The pre-charge forces a ball against a valve seat to keep the valve closed at operating pressures below the pre-charge pressure. Several problems are common with bellows-type gas-lift valves. First, the bellows often loses its pre-charge, causing the valve to fail in the closed position or operate at other than the design goal, and exposure to overpressure causes similar problems. Another common failure is erosion around the valve seat and deterioration of the ball stem in the valve. This leads to partial failure of the valve or at least inefficient production. Because the gas flow through a gas-lift valve is often not continuous at a steady state, but rather exhibits a certain amount of hammer and chatter as the ball rapidly opens and closes, ball and valve seat degradation are common, and lead to gas leakage. Failure or inefficient operation of bellows-type valves leads to corresponding inefficiencies in operation of a typical gas-lift well. In fact, it is estimated that well production is at least 5-15% less than optimum because of valve failure or operational inefficiencies. Fundamentally these difficulties are caused by the present inability to monitor, control, or prevent instabilities, since the valve characteristics are set at design time, and even without failure they cannot be easily changed after the valve is installed in the well.
It would, therefore, be a significant advantage if a system and method were devised which overcame the inefficiency of conventional bellows-type gas-lift valves. Several methods have been devised to place controllable valves downhole on the tubing string but all such known devices typically use an electrical cable or hydraulic pipe disposed along the tubing string to power and communicate with the gas-lift valves. It is, of course, highly undesirable and in practice difficult to use a cable along the tubing string either integral with the tubing string or spaced in the annulus between the tubing string and the casing because of the number of failure mechanisms present in such a system. The use of a cable presents difficulties for well operators while assembling and inserting the tubing string into a borehole. Additionally, the cable is subjected to corrosion and heavy wear due to movement of the tubing string within the borehole. An example of a downhole communication system using a cable is shown in PCT/EP97/01621.
U.S. Pat. No. 4,839,644 describes a method and system for wireless two-way communications in a cased borehole having a tubing string. However, this system describes a communication scheme for coupling electromagnetic energy in a TEM mode using the annulus between the casing and the tubing. This inductive coupling requires a substantially nonconductive fluid such as crude oil or diesel oil in the annulus between the casing and the tubing. The invention described in U.S. Pat. No. 4,839,644 has not been widely adopted as a practical scheme for downhole two-way communication because it is expensive, has problems with brine leakage into the casing, and is difficult to use. Another system for downhole communication using mud pulse telemetry is described in U.S. Pat. Nos. 4,648,471 and 5,887,657. Although mud pulse telemetry can be successful at low data rates, it is of limited usefulness where high data rates are required or where it is undesirable to have complex, mud pulse telemetry equipment downhole. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 4,468,665; 4,578,675; 4,739,325; 5,130,706; 5,467,083; 5,493,288; 5,574,374; 5,576,703; and 5,883,516. Methods and uses of downhole permanent sensors and control systems are described in U.S. Pat. Nos. 4,972,704; 5,001,675; 5,134,285; 5,278,758; 5,662,165; 5,730,219; 5,934,371; 5,941,307.
It is generally known that in a gas-lift well, an increase of compressed gas injected downhole (i.e. lift-gas) does not linearly correspond to the amount of oil produced. More specifically, for any particular well under a particular set of operating conditions, the amount of gas injected can be optimized to produce the maximum oil. Unfortunately, using conventional bellows type valves, the opening pressure of the gas-lift bellows type valves is preset and the primary control of the well is through the amount of gas injected at the surface. Feedback to determine optimum production of the well can take many hours and even days.
It is also generally known that in two-phase flow regimes, such as in a gas-lift well, several flow regimes exist with varying efficiencies (see A. van der Spek and A. Thomas, Neural Net Identification of Flow Regime Using Band Spectra of Flow Generated Sound, SPE 50640, October 1998). However, while operating in a particular flow regime is known to be desirable, it has largely been considered impossible to practically implement.
It would, therefore, be a significant advance in the operation of gas-lift wells if an alternative to the conventional bellows-type valve were provided, in particular, if sensors for determining flow characteristics in the well could work with controllable gas-lift valves and surface controls to optimize fluid flow in a gas-lift well. Generally, it would be a significant advance to be able to detect the flow regime in a two-phase flow conduit and to control the operation to remain in a desirable phase.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein, it is incorporated by reference for background purposes and indicative of the knowledge of one of ordinary skill in the art.
The problems outlined above are largely solved by the system and method in accordance with the present invention for determining a flow regime and controlling the flow characteristics to attain a desirable regime. In a preferred embodiment, a controllable gas-lift well includes a cased wellbore having a tubing string positioned within and longitudinally extending within the casing. An annular space is defined between the casing and the tubing string. In the simplest case a controllable gas-lift valve is coupled to the tubing string to control the gas injection between the annular space and an interior of the tubing string, normally the lowest valve in the lift production tubing. In a more complete and desirable case any or all of the intermediate valves used for unloading and kick-off may be controllable. The controllable gas-lift valve and sensors are powered and controlled from the surface to regulate such tasks as the fluid communication between the annular space and the interior of the tubing and the amount of gas injected at the surface. Communication signals and power are sent from the surface using the tubing and casing as conductors. The power is preferably a low voltage AC current around 60 Hz.
In more detail, a surface computer having a modem imparts a communication signal to the tubing, and the signal is received by a modem downhole connected to the controllable gas-lift valve. Similarly, the modem downhole can communicate sensor information to the surface computer. Further, power is input into the tubing string and received downhole to control the operation of the controllable gas-lift valve. Preferably, the casing is used as the ground return conductor. Alternatively, a distant ground may be used as the electrical return. In a preferred embodiment, the controllable gas-lift valve includes a stepper motor which operates to insert and withdraw a cage trim valve from a seat, regulating the gas injection between the annulus and the interior of the tubing. The ground return path is provided from the controllable gas-lift valve via a packer or a conductive centralizer around the tubing which is in electrical contact with the tubing, and is also in electrical contact with the casing.
In enhanced form, the controllable gas-lift well includes one or more sensors downhole which are preferably in contact with the downhole modem and communicate with the surface computer. In addition to acoustic sensors, sensors such as temperature, pressure, hydrophone, geophone, valve position, flow rate, and differential pressure sensors provide important information about conditions downhole. The sensors supply measurements to the modem for transmission to the surface or directly to a programmable interface controller for determining the flow regime at a given location and operating the controllable gas-lift valve and surface gas injection for controlling the fluid flow through the gas-lift valve.
Preferably, ferromagnetic chokes are coupled to the tubing to act as a series impedance to current flow on the tubing. In a preferred form, an upper ferromagnetic choke is placed around the tubing below the tubing hanger, and the current and communication signals are imparted to the tubing below and the upper ferromagnetic choke. A lower ferromagnetic choke is placed downhole around the tubing with the controllable gas-lift valve electrically coupled to the tubing above the lower ferromagnetic choke, although the controllable gas-lift valve may be mechanically coulped to the tubing below the lower ferromagnetic choke. It is desirable to mechanically place the operating controllable gas-lift valve below the lower ferromagnetic choke so that the borehole fluid level is below the choke.
Preferably, a surface controller (computer) is coupled via a surface master modem and the tubing to the downhole slave modem of the controllable gas-lift valve. The surface computer can receive measurements from a variety of sources, such as the downhole sensors, measurements of the oil output, and measurements of the compressed gas input to the well (flow and pressure). Using such measurements, the computer can compute an optimum position of the controllable gas valve, and more particularly, the optimum amount of the gas injected from the annular space through each controllable valve into the tubing. Additional parameters may be controlled by the computer, such as controlling the amount of compressed gas input into the well at the surface, controlling back pressure on the wells, controlling a porous frit or surfactant injection system to foam the oil, and receiving production and operation measurements from a variety of the wells in the same field to optimize the production of the field.
The ability to actively monitor current conditions downhole, coupled with the ability to control surface and downhole conditions, has many advantages in a gas-lift well. Conduits such as gas-lift wells have four broad regimes of fluid flow, namely bubbly, Taylor, slug and annular flow. The most efficient production (oil produced versus gas injected) flow regime is the Taylor flow regime.
The downhole sensors of the present invention enable the detection of Taylor flow. The above referenced control mechanismsxe2x80x94surface computer, controllable valves, gas input, surfactant injection, etc.xe2x80x94provide the ability to attain and maintain Taylor flow. In enhanced forms, the downhole controllable valves may be operated independently to attain localized Taylor flow.
In the preferred embodiments, all of the gas lift valves in the well are of the controllable type and may be independently controlled. It is desirable to lift the oil column from a point on the borehole as close as possible to the production packer. More specifically, the lowest gas-lift valve is the primary valve in production. The upper gas-lift valves are used for unloading and kick-off of the well during production initiation. In conventional gas-lift wells, these upper valves have bellows pre-set with a 200 psi margin of error to ensure the valves close after set off. This means lift pressure is lost downhole to accommodate this 200 psi loss per valve. Further, such conventional valves often leak and fail to fully close. Use of the controllable valves of the present invention overcomes such shortcomings.
Construction of such a controllable gas-lift well is designed to be as similar to conventional construction methodology as possible. That is, after casing the well, a packer is typically set above the production zone. The tubing string is the fed through the casing into communication with the production zone. As the tubing string is made up at the surface, a lower ferromagnetic choke is placed around one of the conventional tubing string sections for positioning above the downhole packer. In the sections of the tubing string where it is desired, a gas-lift valve is coupled to the string. A pre-assembled pipe joint prepared with the choke and its associated electronics module, and a controllable gas lift valve, may be used to improve efficiency of field operations. In a preferred form, a side pocket mandrel for receiving a slickline insertable and retractable gas-lift valve is used. With such configuration, either a controllable gas-lift valve in accordance with the present invention can be inserted in the side pocket mandrel or a conventional bellows-type valve can be used. Alternatively, the controllable gas-lift valve may be tubing conveyed. When make-up of the tubing string nears completion, a ferromagnetic choke is again placed around an upper joint of the tubing string, this time just below the tubing hanger, or a prefabricated joint with choke already installed may be used. Communication and power leads are then connected through the wellhead feed through to the tubing string below the upper ferromagnetic choke.
In an alternative form, a sensor and communication pod is inserted without the necessity of including a controllable gas-lift valve. That is, an electronics module having pressure, temperature or acoustic, or other sensors, a power supply, and a modem is inserted into a side pocket mandrel for communication to the surface computer using the tubing string and casing as conductors. Alternatively, electronics modules may be mounted directly on the tubing (tubing conveyed) and not be configured to be wireline replaceable. If directly mounted to the tubing an electronic module or a controllable gas-lift valve may only be replaced by pulling the entire tubing string. In an alternative form, the controllable valve can have its separate control, power and wireless communication electronics mounted in the side pocket mandrel of the tubing and not in the wireline replaceable valve. In the preferred form, the electronics are integral and replaceable along with the gas-lift valve. In another form, the high permeability magnetic chokes may be replaced by electrically insulated tubing sections. Further, an insulated tubing hanger in the wellhead may replace the upper choke or such upper insulating tubing sections.
Although the downhole sensors, electronics modules, and valves can be configured in many different ways, the primary function of the components is to determine and regulate the existing flow regime of oil and gas in the tubing string. Sensor measurements are communicated to the surface using the tubing string and the casing as conductors. These measurements are then used to calculate and regulate gas injection, both at the surface and downhole, in order to obtain the desired downhole flow regime.